A detailed history of electrical cardiac stimulators, from the earliest pacemakers capable of delivering electrical stimuli at a fixed rate regardless of the heart's functioning to the modern microprocessor-controlled cardioverting/defibrillating pacemakers capable of delivering measured electrical stimuli based on the haemodynamic state of a patient, is provided in U.S. patent application No. 07/481,364 to K. A. Collins, filed Feb. 16, 1990, and entitled "An Implantable Automatic and Haemodynamically Responsive Cardioverting/Defibrillating Pacemaker." The contents of U.S. patent application No. 07/481,364 are hereby incorporated herein by reference.
The pacemaker of U.S. patent application No. 07/481,364 includes means to overcome many of the problems relating both to the failure to provide necessary electrical therapy, and to the delivery of unnecessary electrical therapy, that arose in earlier systems which relied upon the sensing of blood pressure in the heart as a means for determining the state of cardiac function. It thus also has applicability to the present invention, as will become more apparent below. Similarly, the subjects matter of U.S. patent application No. 07/600,419 to K. A. Collins et al., filed Oct. 19, 1990, and entitled "Implantable Haemodynamically Responsive Cardioverting/Defibrillating Pacemaker," and U.S. patent application No. 07/557,248 to K. A. Collins et al., filed Jul. 24, 1990, and entitled "An Implantable Automatic and Hemodynamically Responsive Cardioverting/Defibrillating Pacemaker With Means For Minimizing Bradycardia Support," also are applicable to the present invention and are therefore also incorporated herein by this reference. The three aforementioned United States patent applications are all assigned to the assignee of the present invention. A brief summary of each of these applications is provided below.
U.S. patent application No. 07/481,364 discloses a cardioverting/defibrillating pacemaker with the ability to sense and respond to haemodynamic compromise. The degree of haemodynamic compromise is determined with the aid of an appropriate algorithm by comparing the derived ventricular filtered peak-to-peak amplitude or derived ventricular peak pressure with programmed values for these parameters.
U.S. patent application No. 07/600,419 discloses, among other innovative concepts, a novel method and means for monitoring both an electrocardiogram (ECG) signal system and a haemodynamic signal system whereby the functioning of the pacemaker device is altered if there is a failure in either of the monitored systems. The disclosed invention ensures that the sensing circuitry for performing the monitoring tasks is operating within defined limits and deactivates either one of the systems that is not within the defined limits. The disclosed invention also ensures that the gain of the sensing amplifiers is maintained within proper limits; and, it uses the unaffected sensing system alone to determine how the device should function in the event one of the systems is not operating within the prescribed limits.
U.S. patent application No. 07/557,248 is directed to a method and apparatus for minimizing the energy required for bradycardia support pacing in order to lengthen battery life. This energy minimization is achieved by sensing the right ventricular pressure (RVP) to determine if a response to bradycardia support pacing is evoked. The bradycardia pacing voltage is reduced to the lowest level it can have while still being able to "capture" the heart. The RVP waveform is monitored and the number of pressure pulses evoked in response to a number of pacing pulses is counted. If at least X pressure pulses are sensed in response to Y pacing pulses, capture is assumed, where X is less than or equal to Y. There are several other embodiments disclosed and claimed which also depend upon the monitoring of right ventrical pressure.
The foregoing Collins and Collins et al. applications all require that haemodynamic inputs be provided to the pacemakers employed therein. Endocardial pacing lead systems equipped with haemodynamic pressure sensors are generally used for this purpose. U.S. Pat. Nos. 4,566,456 to G. Koning et al., 4,708,143 to E. A. Schroeppel, and 4,967,755 to P. J. Pohndorf are representative of patents that disclose endocardial pacing lead systems which are provided with haemodynamic pressure sensors. A brief discussion of each of these patents follows.
The Koning et al. U.S. Pat. No. 4,566,456 discloses a pacer system including a pacemaker and an endocardial sensing/pacing lead. The sensing/pacing lead includes a pressure sensor therein which comprises a piezo-resistive micro-electronic transducer on a chip which is embedded within an elastomeric material. The transducer is secured within an opening in a tubular sheath of the lead. It is mounted on a glass carrier, and four insulated sensor wire conductors are coupled to the glass carrier for connection to a bridge comprised of variable piezo-resistors of the transducer. The four wires extend within the interior of the lead to the proximal end thereof, and a multi-terminal connector is employed at the proximal end of the lead to connect the four wires of the pressure sensor and the electrical sensing/pacing wires of the lead to the pacemaker.
The Schroeppel U.S. Pat. No. 4,708,143 discloses a pacer system including a pacemaker and an endocardial sensing/pacing lead. The sensing/pacing lead includes a pressure sensor incorporated in the form of a piezoelectric bimorph located in the elastomeric body of the lead, between the outer and inner walls thereof. The bimorph includes a pair of piezoelectric ceramic sheets separated by a brass shim. A pair of insulated wires, located in the body of the lead, connect the upper and lower surfaces of the bimorph to corresponding terminals at the proximal end of the lead, for connection to the pacemaker. Alternative pressure sensors in the form of piezoelectric strips constructed of thin film polymers, mounted in the body of the distal portion of the lead in various configurations, are also disclosed.
The Pohndorf U.S. Pat. No. 4,967,755 also discloses an endocardial sensing/pacing lead which includes a haemo-dynamic pressure sensor therein, as well as tip and ring pacing/sensing electrodes. The pressure sensor includes a metallic case, in the shape of one-half of a cylinder, that is positioned within and mechanically and electrically coupled to the ring electrode. A resilient, insulating, elastomeric material, also in the shape of one-half of a cylinder and having a metallic, axially extending tube centered therein and passing therethrough, provides access for a stylet to pass between the sensor body and the ring electrode for fixing the tip electrode in place. The two half-cylinders are positioned within the ring electrode, with their outer planar diameter surfaces in abutment with one another so that, together, they form a full cylinder. The ring electrode is provided with apertures which allow the pressure of surrounding blood to be transmitted to and through the elastomeric material to the outer surface of a diaphram that is welded around an opening in the outer planar surface of the metallic sensor case. A piezoelectric crystal fixed to the inner surface of the diaphram and coupled to electronic circuitry within the metallic sensor case provides an output signal representative of the haemo-dynamic pressure. This output signal is provided across (a) one output lead of the sensor, which lead is connected via one insulated wire of a multi-filar coil wire to one surface of a multiple proximal connector, and (b) the metallic case of the sensor, which case is connected via the ring electrode and the remaining wires of that coil wire to another surface of the proximal connector.
Although the foregoing pressure sensor equipped endocardial pacing lead systems are capable of performing their intended functions, the pressure sensing portions thereof are less than optimal with respect to sensor sensitivity, sensor output, durability of the pressure sensor, and efficient use of connective wiring to prevent interference with stylet insertion during implantation.
It is, therefore, a primary object of the present invention to provide an endocardial lead having an improved integral haemodynamic pressure sensor therein.
It is another object of the present invention to provide an improved endocardial lead that incorporates therein sensing/pacing and defibrillating electrodes in combination with an improved haemodynamic pressure sensor.
It is a further object of the present invention to provide an improved endocardial lead that incorporates therein bipolar sensing/pacing electrodes in combination with a haemodynamic pressure sensor.
Further objects and advantages of the present invention will become apparent as the following description proceeds.